A reliable supply of sterile instruments and supplies is important to modem medical practice. Various types of apparatus are known for sterilizing reusable goods within a hospital setting. Among the best known is the steam autoclave, which uses high temperature and high pressure steam to render medical goods sterile. When used as a sterilizing gas, steam is fast and effective, but its use requires high temperatures. Goods that cannot withstand autoclaving temperatures can be sterilized with sterilizers using biocidal gases such as ethylene oxide or hydrogen peroxide. Some types of medical goods are also suited to being sterilized by a liquid bath.
Whatever the selected method and sterilizing agent, an important part of the process of providing sterile goods is verifying that conditions necessary for sterilizing are met. Two broad classes of indicators are typically used for such verification. The first class, the so-called biological indicators, includes devices that employ viable spores of particularly hardy stains of bacteria. After a sterilizing cycle, the biological indicator is placed in an environment conducive to bacterial growth. If no growth occurs, an effective cycle is presumed. The second class is the so-called chemical indicators; these devices include a chemical compound that undergoes some sort of change (e.g., color, physical or chemical state, etc.) when subjected to a set of pre-determined conditions of sterilization within a sterilizing chamber. If the change occurs in the indicator, the sterilization cycle is presumed effective.
One change chemical indicators can undergo is a change in state. An indicator can, for example, melt. Devices that incorporate such an indicator can be made useful for remote monitoring such that the indicator can be assessed at a short distance without the need for its visual inspection. Melting of the indicating substance can, for instance, permit a physical rearrangement of a magnetostrictive element within the indicator which can be detected remotely by well-known electronic techniques.
Remote monitoring is an advantage because it is typical to enclose medical goods within a package or wrapping prior to sterilization. For example, the surgical instruments desired for a particular procedure can be laid out on a tray in a convenient layout and wrapped as an assembly in material that is porous to the sterilizing gas but nonporous to bacteria. The wrapped tray can be sterilized and maintained as a wrapped assembly until delivered to the surgical suite. Often a chemical indicator is attached to the outside of the wrapped assembly, and a change in the indicator to show that the exterior was subjected to sterilizing conditions is taken as a proxy showing that instruments in the interior of the wrapped assembly have also been rendered sterile. It may be preferred, however, to assess the sterilization of the goods by direct measurement of the conditions within the enclosed packaging material. While it is possible to include another chemical indicator within the wrapped assembly, unless adapted for remote monitoring its condition may not be reliably known until the tray is unwrapped in the surgical suite. At that point, it is inconvenient to learn that the goods are not sterile. It is particularly inconvenient if nonsterile goods from a newly unwrapped tray are placed inadvertently within the sterile field before being noticed; in such case the process of preparing the suite for surgery may have to begin again.
Known indicators adapted for remote monitoring generally have the disadvantage of being capable of only a single use, and mechanical components contained in such monitors are substantially more expensive than standard indicators.